i18n(fr): add missing French locale keys

[ThomasRbn]

Summary

• Add missing French translations across the dashboard locale files so the FR UI no longer falls back to English.
• Cover newly needed strings for jobs, migration, project settings, billing, overview, and related screens.
• Update existing FR locale files (billing, dashboard, settings, widgets, etc.) and add jobs.json / migration.json where keys were missing.

Test plan

• Switch the dashboard locale to French
• Spot-check Jobs, Migration, Project Settings, Billing, Overview, Onboarding, and Widgets for English fallbacks
• Confirm key screens still render correctly (no missing-key placeholders)
• Compare a few strings against the English source locales for accuracy/tone